Currently, a touch detecting assembly (i.e., a touch screen) has been widely applied in electronic apparatuses, such as mobile phones, PDA (personal digital assistant), GPS (global positioning system), PMP (such as MP3, MP4) and panel computers. The touch screen, which has advantages of simple, convenient and humanized touch operations, will be a best human-computer interaction interface and be widely applied in portable apparatuses.
A capacitance touch detecting assembly is generally divided into two types: self-capacitance type and mutual-capacitance type. A conventional single-layer self-capacitance touch screen comprises a plurality of bar scan electrodes made from ITO (indium tin oxides) on a glass surface. As a conductive substance with a fixed resistivity, ITO has a good material coherence, which has been proved from a linearity of a resistor screen. The plurality of electrodes and surroundings such as a ground or a circuit constitute two electrodes of a capacitor respectively. One capacitor is connected into the circuit in parallel by a touch on the one capacitor by a finger or a touch pen, thus changing a total capacitance of a bar scan line. During a scanning process, each induction component is scanned by a control IC (integrated circuit) in a specific scan mode, and a touch position is determined according to a variation of capacitances between before the scanning and after the scanning, so as to realize the human-computer interaction. Generally, the capacitance touch detecting assembly works together with a TFT (thin film transistor) and a LCD (liquid crystal display) on which the capacitance touch detecting assembly is disposed.
FIG. 1 shows a conventional self-capacitance type touch detecting assembly. The self-capacitance type touch detecting assembly comprises a plurality of induction units 100′ and 200′ which have a diamond structure and are located in two different layers. A scanning is conducted along an X axis and a Y axis respectively, and if a capacitance variation of a certain intersection point exceeds a predetermined range, the intersection point is made as a touch point. Although a linearity of the self-capacitance type touch detecting assembly is good, ghost touch points still appear frequently, and thus it is difficult to realize a multipoint touch. In addition, since a double-layer screen is used, the structure is complicated and the cost is increased. Moreover, under a condition of a slight capacitance variation, the diamond structure may cause a coordinate drift, that is, the diamond structure may be easily affected by an external factor.
FIG. 2a shows another conventional self-capacitance type touch detecting assembly. The self-capacitance type touch detecting assembly uses a triangular screen structure. The self-capacitance type touch detecting assembly comprises: a substrate 300′, a plurality of triangular induction units 400′ disposed on the substrate 300′, and a plurality of electrodes 500′ connected with the triangular induction units 400′ respectively. As shown in FIG. 2a, an ellipse 600′ represents a finger. FIG. 2b shows a detecting principle of the self-capacitance type touch detecting assembly shown in FIG. 2a. As shown in FIG. 2b, an ellipse represents a finger which contacts with two adjacent triangular induction units, S1 represents a contact area between the finger and one of the two adjacent triangular induction units, and S2 represents a contact area between the finger and the other. Provided that an origin of coordinate is located at the lower-left corner, an X coordinate may be obtained by X=S2/(S1+S2)*P, where P is a resolution ratio. When the finger moves rightwards, because S2 does not increase linearly, there is a deviation of the X coordinate. It may be known from the detecting principle that a single end detecting is conducted for the conventional triangular induction unit, that is, the detecting is conducted only from one direction, and coordinates in the two directions are calculated by an algorithm. Although the self-capacitance type touch detecting assembly has a simple structure, an induction capacitance of the screen is not optimized, so that the capacitance variation is small, thus reducing a signal-to-noise ratio. In addition, because each induction unit has a triangular shape, when the figure moves horizontally, the contact area may not increase linearly, thus causing the deviation of the X coordinate and a poor linearity accordingly.
In addition, because the capacitance variation of a conventional capacitance induction unit is small to a femtofarad order of magnitude, a measure circuit needs to satisfy a higher requirement because of an existence of a stray capacitance. Moreover, because the stray capacitance may vary because of many factors, such as temperature, position, and distribution of internal and external electric field, the stray capacitance may interfere with or even bury a tested capacitance signal. In addition, for a single-layer capacitance, because the induction capacitance may be seriously interfered by an influence of a level signal Vcom, which is used for preventing a liquid crystal of a LCD screen from aging.